The invention relates to a method of and an apparatus (circuit) for protecting the thyristors of a pulse generator of a pulse operated electrostatic precipitator.
In a pulse operated electrostatic precipitator the voltage pulse is provided by triggering a switch element, usually a thyristor or a circuit consisting of series and/or parallel coupled thyristors. When the pulse has reached its peak the current in the thyristors ceases as during the pulse decay, the current flows in return diodes which are coupled in parallel with the thyristors. Once the current in the thyristors has ceased for a certain period of time, the recovering time, they become non-conductive in their forward direction until they are retriggered to provide a new pulse.
If a spark-over occurs in the electrostatic precipitator after the current in the thyristors has ceased, but before the recovery time has elapsed, the thyristors will become forward biased and current will be drawn through the thyristors even though they are only partly conductive. In such a case the current is concentrated in separate, still partly conducting areas of the thyristor semi-conductor chips with the result that these are consequently overloaded and possibly damaged or destroyed.
Nos. EP-A-0066950 and EP-A-014522 describe methods by which retriggering of the thyristors of the pulse generator is established when spark-overs in the electrostatic precipitator are detected. Such a retriggering means that the thyristors can take over the current again without the danger of overload, provided that the retriggering signal has been established by the time the current shifts from the return diodes to the thyristors.
In the narrow interval of time from the moment that the current has shifted from the thyristors to the return diodes and for a few micro-seconds afterwards, due to unavoidable reaction times in the retriggering system, it is difficult to ensure that the retriggering signal can be established by the time that the current shifts back to the thyristors as a consequence of precipitator spark-over.
This is particularly so in extreme operating situations where a low voltage pulse height is used in the electrostatic precipitator simultaneously with use of a high DC voltage. Under these circumstances the time from the occurrence of the spark-over until the current will try to flow in the thyristors will be fractions of a micro-second.
As the time required for detecting the spark-over and generating the retriggering signal typically is one or two micro-seconds, the retriggering signal will consequently be too late.